FTP: perform active connections non-blocking

1- Two new error codes are introduced.

CURLE_FTP_ACCEPT_FAILED to be set whenever ACCEPTing fails because of
FTP server connected.

CURLE_FTP_ACCEPT_TIMEOUT to be set whenever ACCEPTing timeouts.

Neither of these errors are considered fatal and control connection
remains OK because it could just be a firewall blocking server to
connect to the client.

2- One new setopt option was introduced.

CURLOPT_ACCEPTTIMEOUT_MS

It sets the maximum amount of time FTP client is going to wait for a
server to connect. Internal default accept timeout is 60 seconds.

tests/data/test1207

@@ -36,12 +36,6 @@ FTP PORT and 421 on download
 <strippart>
 s/^EPRT \|1\|(.*)/EPRT \|1\|/
 </strippart>
-
-# The protocol part does not include QUIT simply because the error is
-# CURLE_OPERATION_TIMEDOUT (28) which is a generic timeout error without
-# specificly saying for which connection it concerns, and for timeouts libcurl
-# marks the control channel as "invalid". As this test case times out for the
-# data connection it could still use the control channel.
 <protocol>
 USER anonymous
 PASS ftp@example.com
@@ -50,9 +44,10 @@ EPRT |1|
 TYPE I
 SIZE 1207
 RETR 1207
+QUIT
 </protocol>
 <errorcode>
-28
+10
 </errorcode>
 </verify>
 </testcase>